Flash generators employ a capacitor bank for storing an amount of energy which can be discharged through a flash tube so as to generate a discharge and a flash light. During the discharge, plasma is formed inside the flash tube where the character of the plasma also determines the character of the flash light. A flash tube is dimensioned, as far as is possible, to give constant light characteristics throughout its entire dynamic range. A series of parameters, such as the physical dimensions of the flash tube, the gas pressure, the gas mixture and the composition of the electrodes is adapted in order, together with the operating voltage and capacities of the flash unit, to provide the optimal conditions for constant light characteristics within the dynamic range of the electron flash tube.
In a camera, flash tube synchronization is defined as synchronizing the firing of the flash tube with the opening of the shutter admitting light to photographic film or image sensor. One type of flash tube synchronization is FP-sync, Flat Peak. FP-sync is used with flash tubes designed specifically for use with focal-plane shutters. A focal-plane shutter uses two shutter curtains that run horizontally or vertically across the image sensor plane. For slower shutter speeds, the first curtain opens, and after the required time with the shutter open, the second curtain closes the aperture in the same direction. Faster shutter speeds are achieved by the second curtain closing before the first one has fully opened. This results in a slit that travels across the image sensor. Faster shutter speeds simply require a narrower slit, as the speed of travel of the shutter curtains is not normally varied. Using this technique, modern SLR cameras are capable of shutter speeds of up to 1/2000, 1/4000 or even 1/8000 of a second.
When using a focal plane shutter, although each part of the image sensor is exposed for the rated exposure time, the image sensor is exposed by a slit which moves across the image sensor in a time, the X-sync speed. The X-sync speed may be of the order of 1/250. Although the exposure of each part of the image sensor may be 1/2000, the last part of the image sensor is exposed later by the X-sync time than the first part of the image sensor. If the flash tube discharge for a shorter time than the X-sync speed only parts of the image sensor will be illuminated. Flash tubes that discharge during the entire X-sync time will result in that the entire image sensor will be illuminated even at higher shutter speeds.
When the flash tube is discharged for a long time with constant energy required to illuminate the entire image sensor, the flash tube can be considered as a fixed light source. One problem with a flash tube that is designed for a discharge with a much larger energy and a shorter duration is that the plasma formed in the flash tube will not be constant when the flash tube is discharged for a long time. This results in a varying energy from the flash tube every time the flash tube is discharged.
FIG. 1 illustrates the energy level for different discharges when the flash tube is discharged for a long time. As can be seen from FIG. 1, energy will vary from one flash discharge (a) to another flash discharge (b). If the flash discharges are shorter with higher energy levels for the different flashes, the energy variation of the light would not vary to the same extent from the different flash discharges.
FIG. 2 illustrates another problem when discharging flash tubes for longer times at lower energy levels. As illustrated in FIG. 2, it takes longer time to achieve constant amplitude over time compared to a flash with higher amplitude over time.
Thus, when increasing the discharge time and lowering the energy from the flash tube, the discharge gives a low quality of light from the flash tube. This since a flash tube which is adapted for discharges of high energy amounts during short times does not give repeatable stable discharges at low energy amounts and longer discharge times. Yet another problem when discharging during longer times is that that the energy level variations are considerably higher than the variations which occur on discharges during shorter times and higher energy amounts.
Consequently, there is a need for an improved solution for discharging flash tubes, which solution solves or at least mitigates at least one of the above mentioned problems.